Tomographic images, etc. of an examinee which are scanned by an X-ray CT (Computed Tomography) apparatus, an MRI (Magnetic Resonance Imaging) apparatus, an ultrasonic apparatus, etc. have been hitherto known as images used for medical diagnosis. There has been developed a computer-aided detection apparatus (Computer-Aided Detection; hereinafter referred to as CAD) in which a medical image as described above is analyzed by using a computer to detect lesion candidates from shade and shadow of the medical image and present the lesion candidates to a medical doctor. CAD automatically detects an image region estimated as a lesion site (hereinafter referred to as lesion candidate region) on the basis of a form characteristic or a density characteristic of the lesion site, and it reduces a labor imposed on the medical doctor.
Furthermore, when a large number of cases are required to be read like health check or the like, there is an operator's requirement of extracting and displaying lesion candidates of plural desired sizes at a time through a series of processing to efficiently perform diagnosis. For example, polyps in a colon region have a characteristic feature, but have various sizes. In general, lesion candidates as medical treatment targets are equal to 5 mm or more in size, and lesion candidates of 10 mm or more have a high risk that they become colon cancers. For example, Patent Document 1 discloses a method of extracting lesion candidates by making an evaluation using a feature amount representing the form of a curved surface (shape index) for a medical image.